Work-to-sleep ratio as a novel marker of NAFLD risk: evidence from U.S. and Korean national cohorts.

backgroundNonalcoholic fatty liver disease (NAFLD), now redefined as metabolic dysfunction-associated steatotic liver disease (MASLD), affects 25-30% of adults globally and is driven by metabolic dysregulation. Concurrently, prolonged work hours and insufficient sleep are increasingly prevalent, yet their combined relationship with NAFLD risk remains unexplored. The Work-to-Sleep Ratio (WSR), quantifying the balance between occupational time and sleep recovery, may serve as a novel behavioral marker of NAFLD risk. Unlike work hours or sleep duration alone, WSR integrates these two interdependent behaviors into a single metric, capturing the balance between occupational demand and physiological recovery.

methodsThis cross-sectional study utilized nationally representative data from the U.S. NHANES (2017-2023; n = 3,935) and Korean KNHANES (2019-2020 and 2022-2023; n = 10,729) cohorts. Multivariable logistic regression models with sequential covariate adjustment were employed to examine WSR-NAFLD associations. Restricted cubic spline analyses explored nonlinear relationships, while mediation analyses examined the potential involvement of adiposity (BMI), inflammation (CRP), and dyslipidemia (NHHR). Subgroup analyses assessed effect modification by demographic and clinical factors.

resultsWSR exhibited nonlinear associations with NAFLD risk in both cohorts. In NHANES, NAFLD risk increased with rising WSR and attenuated at higher levels. In contrast, in KNHANES, higher WSR was associated with progressively increased NAFLD risk that plateaued at elevated ratios. Associations were stronger and more consistent in KNHANES (fully adjusted OR per unit increase in WSR 1.57, 95% CI 1.38-1.78). Subgroup analyses revealed stronger associations in younger adults and metabolically vulnerable groups. Mediation analyses identified adiposity (BMI) as accounting for the largest proportion of the observed WSR-NAFLD association, with secondary roles for inflammation (CRP) and dyslipidemia (NHHR).

conclusionWSR may serve as a useful behavior-based marker associated with NAFLD in nationally representative U.S. and Korean cohorts, with associations most evident among younger adults. Adiposity appeared to play a central role in this association, with additional contributions from inflammation and dyslipidemia. These findings highlight WSR as a simple time-based indicator for identifying populations at elevated NAFLD risk. Longitudinal studies are needed to confirm these associations and clarify temporal relationships.